[OpenMP][libomp] Detect if test compiler has omp.h

omp50_taskdep_depobj.c relies on the test compiler's omp.h file.
If the test compiler does not have an omp.h file, then use the one
within the build tree.

Fixes: https://github.com/llvm/llvm-project/issues/56820
Differential Revision: https://reviews.llvm.org/D131000